APHA Method 2130: Standard Methods for the Examination of Water and Wastewater

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Document Name: APHA Method 2130: Standard Methods for the

Examination of Water and Wastewater

CFR Section(s) : 40 CFR 141.121

Standards Body: American Public Health Association

Standard

For the

Examination of
Water and
Wastewater

: mm

Imm^mmmm^mmm^

Prepared and published jointly by:

American Public Health Association

American Water Works Association

Water Environment Federation

joint Editorial Board

Arnold E. Greenberg, APHA, Chairman

Lenore S. Qesceri, WEF

Andrew D. Eaton, AWWA

Managing Editor
Mary Ann H. Franson

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American Public Health Association.

Standard methods for the examination of water and wastewater.
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2-8

PHYSICAL & AGGREGATE PROPERTIES (2000)

2130 TURBIDITY*

2130 A. Introduction

1. Sources and Significance

Clarity of water is important in producing products destined
for human consumption and in many manufacturing uses. Bev-
erage producers, food processors, and treatment plants drawing
on a surface water supply commonly rely on coagulation, settling,
and filtration to insure an acceptable product. The clarity of a
natural body of water is a major determinant of the condition
and productivity of that system.

Turbidity in water is caused by suspended matter, such as clay,
silt, finely divided organic and inorganic matter, soluble colored
organic compounds, and plankton and other microscopic organ-
isms. Turbidity is an expression of the optical property that causes
light to be scattered and absorbed rather than transmitted in
straight lines through the sample. Correlation of turbidity with
the weight concentration of suspended matter is difficult because
the size, shape, and refractive index of the particulates also affect
the light-scattering properties of the suspension. Optically black
particles, such as those of activated carbon, may absorb light
and effectively increase turbidity measurements.

2. Selection of Method

Historically, the standard method for determination of tur-
bidity has been based on the Jackson candle turbidimeter 1 ; how-
ever, the lowest turbidity value that can be measured directly on
this instrument is 25 units. Because turbidities of treated water

Approved by Standard Methods Committee. 1988.

usually fall within the range of to 1 unit, indirect secondary
methods also were developed to estimate turbidity. Unfortu-
nately, no instrument could duplicate the results obtained on the
Jackson candle turbidimeter for all samples. Because of funda-
mental differences in optical systems, the results obtained with
different types of secondary instruments frequently do not check
closely with one another, even though the instruments are pre-
calibrated against the candle turbidimeter.

Most commercial turbidimeters available for measuring low
turbidities give comparatively good indications of the intensity
of light scattered in one particular direction, predominantly at
right angles to the incident light. These nephelometers are un-
affected relatively by small changes in design parameters and
therefore are specified as the standard instrument for measure-
ment of low turbidities. Nonstandard turbidimeters, such as for-
ward-scattering devices, are more sensitive than nephelometers
to the presence of larger particles and are useful for process
monitoring.

A further cause of discrepancies in turbidity analysis is the use
of suspensions of different types of particulate matter for the
preparation of instrumental calibration curves. Like water sam-
ples, prepared suspensions have different optical properties de-
pending on the particle size distributions, shapes, and refractive
indices. A standard reference suspension having reproducible
light-scattering properties is specified for nephelometer calibra-
tion.

Because there is no direct relationship between the intensity
of light scattered at a 90° angle and Jackson candle turbidity,
there is no valid basis for the practice of calibrating a nephelom-
eter in terms of candle units. To avoid misinterpretation, report

TURBIDITY (2130)/Nephelometric Method

2-9

the results from nephelometric measurements as nephelometric
turbidity units (NTU).

Its precision, sensitivity, and applicability over a wide turbidity
range make the nephelometric method preferable to visual meth-
ods. The Jackson candle method has been eliminated from the
1 7th and subsequent editions of Standard Methods.

3. Storage of Sample

Determine turbidity on the day the sample is taken. If longer
storage is unavoidable, store samples in the dark for up to 24 h.

Do not store for long periods because irreversible changes in
turbidity may occur. Vigorously shake all samples before ex-
amination.

4. References

l.AM E R I C A N P U B L I C H E A LT H A SSOC 1 AT ION. A M E Rl C A N W AT E R WO RK S

Association & Water Pollution Control Federation. 1985.
Standard Methods for the Examination of Water and Wastewater,
16th ed. American Public Health Assoc. Washington, D.C.

2130 B. Nephelometric Method

1. General Discussion

a. Principle: This method is based on a comparison of the
intensity of light scattered by the sample under defined conditions
with the intensity of light scattered by a standard reference sus-
pension under the same conditions. The higher the intensity of
scattered light, the higher the turbidity. Formazin polymer is
used as the reference turbidity standard suspension. It is easy to
prepare and is more reproducible in its light-scattering properties
than clay or turbid natural water. The turbidity of a specified
concentration of formazin suspension is defined as 40 nephelo-
metric units. This suspension has an approximate turbidity of 40
Jackson units when measured on the candle turbidimeter; there-
fore, nephelometric turbidity units based on the formazin prep-
aration will approximate units derived from the candle turbidi-
meter but will not be identical to them.

b. Interference: Turbidity can be determined for any water
sample that is free of debris and rapidly settling coarse sediments.
Dirty glassware, the presence of air bubbles, and the effects of
vibrations that disturb the surface visibility of the sample will
give false results. "True colore 1 that is, water color due to dis-
solved substances that absorb light, causes measured turbidities
to be low. This effect usually is not significant in the case of
treated water.

2. Apparatus

a. Turbidimeter consisting of a nephelometer with a light source
for illuminating the sample and one or more photoelectric de-
tectors with a readout device to indicate intensity of light scat-
tered at 90° to the path of incident light. Use a turbidimeter
designed so that little stray light reaches the detector in the
absence of turbidity and free from significant drift after a short
warm up period. The sensitivity of the instrument should permit
detecting turbidity differences of 0.02 NTU or less in waters
having turbidity of less than 1 NTU with a range from to 40
N T U . S e ve ra I r a n ge s a r e ne ce s s a r y to o b ta i n b o t h a d e q u a te cov-
erage and sufficient sensitivity for low turbidities.

Differences in turbidimeter design will cause differences in
measured values for turbidity even though the same suspension
is used for calibration. To minimize such differences, observe
the following design criteria:

I) Light source— Tungsten-filament lamp operated at a color
temperature between 2200 and 3000°K.

2) Distance traversed by incident light and scattered light within
the sample tube— Total not to exceed 10 cm.

3) Angle of light acceptance by detector— Centered at 90° to
the incident light path and not to exceed ±30° from 90°. The
detector, and filter system if used, shall have a spectral peak
response between 400 and 600 nm.

b. Sample tubes, clear colorless glass. Keep tubes scrupulously
clean, both inside and out, and discard when they become scratched
or etched. Never handle them where the light strikes them. Use
tubes with sufficient extra length, or with a protective case, so
that they may be handled properly. Fill tubes with samples and
standards that have been agitated thoroughly and allow sufficient
time for bubbles to escape.

3. Reagents

cl Turbidity-free water: Turbidity-free water is difficult to ob-
tain. The following method is satisfactory for measuring turbidity
as low as 0.02 NTU.

Pass distilled water through a membrane filter having preci-
sion-sized holes of 0.2 [xm; ;: the usual membrane filter used for
bacteriological examinations is not satisfactory. Rinse collecting
flask at least twice with filtered water and discard the next 200
ml,

Some commercial bottled demineralized waters are nearly par-
ticle-free. These may be used when their turbidity is lower than
can be achieved in the laboratory. Dilute samples to a turbidity
not less than 1 with distilled water.

/>. Stock turbidity suspension:

1) Solution I— Dissolve 1.000 g hydrazine sulfate (Caution:
Carcinogen; avoid inhalation, ingestion, and skin contact.),
(NIL) ? TLSO.,, in distilled water and dilute to 100 mL in a vol-
umetric flask.

2) Solution II— -Dissolve 10.00 g hexamethylenetetramine,
(CH 2 ) ft N„ in distilled water and dilute to 100 mL in a volumetric
flask".

3) In a 100-mL volumetric flask, mix 5.0 mL Solution I and
5.0 mL Solution II. Let stand 24 h at 25 ± 3°C, dilute to mark,
and mix. The turbidity of this suspension is 400 NTU.

4) Prepare solutions and suspensions monthly.

* Nuclepore Corporation. 7035 Commerce Circle. Plcasanton. Calif., or equiva
lent.

2-10

PHYSICAL & AGGREGATE PROPERTIES (2000)

c. Standard turbidity suspension: Dilute 10.00 mL stock tur-
bidity suspension to 100 mL with turbidity-free water. Prepare
daily. The turbidity of this suspension is defined as 40 NTU.

d. Alternate standards: As an alternative to preparing and di-
luting formazin, use commercially available standards such as
styrene divinylbenzene beadst if they are demonstrated to be
equivalent to freshly prepared formazin.

e. Dilute turbidity standards: Dilute portions of standard tur-
bidity suspension with turbidity-free water as required. Prepare
daily.

4. Procedure

a. Turbidimeter calibration: Follow the manufacturer's oper-
ating instructions. In the absence of aprecalibrated scale, prepare
calibration curves for each range of the instrument. Check ac-
curacy of any supplied calibration scales on a precalibrated in-
strument by using appropriate standards. Run at least one stand-
ard in each instrument range to be used. Make certain that
turbidimeter gives stable readings in all sensitivity ranges used.
High turbidities determined by direct measurement are likely to
differ appreciably from those determined by the dilution tech-
nique, 11 Ac.

b. Measurement of turbidities less than 40 NTU: Thoroughly
shake sample. Wait until air bubbles disappear and pour sample
into turbidimeter tube. When possible, pour shaken sample into
turbidimeter tube and immerse it in an ultrasonic bath for 1 to
2 s, causing complete bubble release. Read turbidity directly from
instrument scale or from appropriate calibration curve.

c. Measurement of turbidities above 40 NTU: Dilute sample
with one or more volumes of turbidity-free water until turbidity
falls between 30 and 40 NTU. Compute turbidity of original
sample from turbidity of diluted sample and the dilution factor.
For example, if five volumes of turbidity-free water were added
to one volume of sample and the diluted sample showed a tur-
bidity of 30 NTU, then the turbidity of the original sample was
180 NTU.

d. Calibrate continuous turbidity monitors for low turbidities
by determining turbidity of the water entering or leaving them,
using a laboratory-model turbidimeter. When this is not possible,
use an appropriate dilute turbidity standard, 1! 3e. For turbidities
above 40 NTU use undiluted stock solution.

5. Calculation

6. Interpretation of Results

a. Report turbidity readings as follows:

Nephelometric turbidity units (NTU)

A x (B + C)
C

where:

A — NTU found in diluted sample.

B — volume of dilution water, mL, and

C = sample volume taken ('or dilution, mL.

Report to the

Turbidity Range

Nearest

NTU

NTU

0-1.0

0.05

1-10

0.1

10-40

1

40-100

5

100-400

10

400-1000

50

>1000

100

t AMCO-AEPA-1 Standard. Advanced Polymer Systems, 3696 C Haven Ave.
Redwood Citv, Calif.

b. For comparison of water treatment efficiencies estimate
turbidity more closely than is specified above. Uncertainties and
discrepancies in turbidity measurements make it unlikely that
two or more laboratories will duplicate results on the same sam-
ple more closely than specified.

7. Bibliography

Whipple, G.C. & D.D. Jackson. 1900. A comparative study of the
methods used for the measurement of turbidity of water. Mass. Inst.
Technol. Quart. 13:274.

American Public Health Association. 1901. Report of Committee
on Standard Methods of Water Analysis. Pub. Health Papers &
Rep. 27:377.

Wells, P. V. 1922. Turbidimetry of water. J. Amer. Water Works Assoc.
9:488.

Baylis, J.R. 1926. Turbidimeter for accurate measurement of low tur-
bidities. Ind. Eng.Chem. 18:311.

Wells, P. V. 1927. The present status of turbidity measurements. Chem.
Rev. 3:331.

Baylis, J.R. 1933. Turbidity determinations. Water Works Sewage 80:125.

Rose, H.E. & H.B. Lloyd. 1946. On the measurement of the size
characteristics of powders by photo-extinction methods. ./. Soc. Chem.
Ind. (London) 65:52 (Feb.); 65:55 (Mar,).

Rose, H.E. & C.C.J. French. 1948. On the extinction coefficient: Par-
ticle size relationship for fine mineral powders. J. Soc. Chem. hid.
(London) 67:283.

Gillett, T.R., P.F. Meads & A.L. Holven. 1949. Measuring color
and turbidity of white sugar solutions. Anal. Chem. 21:1228.

Jullander, I. 1949. A simple method for the measurement of turbidity.
Acta Chem. Scand. 3:1309.

Rose, FI.E. 1950. Powder-size measurement by a combination of the
methods of nephelometry and photo-extinction. /. Soc. Chem. Ind.
(London) 69:266.

Rose, H.E. 1950. The design and use of photoextinction sedimentom-
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Brice. B.A., M. Halwer & R. Speiser. 1950. Photoelectric light-
scattering photometer for determining high molecular weights. J.
Opt. Soc. Amer. 40:768.

Knight, A.G. 1950. The measurement of turbidity in water. J. Inst.
Water Eng. 4:449.

Hanya, T. 1950. Study of suspended matter in water. Bull. Chem. Soc.
Jap. 23:216.

Jullander, I. 1950. Turbid ime trie investigations on viscose. Svensk
Papperstidn. 22:1.

Rose, H.E. 1951. A reproducible standard for the calibration of turbi-
dimeters. J. Inst. Water Eng. 5:310.

Aitken, R.W. & D. Mercer. 1951. Comment on 'The measurement
of turbidity in water. 11 J. Inst. Water Eng. 5:328.

ODOR (21 50)/lntroduction

2-11

Rose. H.E. 1951. The analysis of water by the assessment of turbidity.
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matter in stream water. J. Amer. Water Works Assoc. 56:1194.

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